Thermophilic Anaerobic Digestion: An Advancement towards Enhanced Biogas Production from Lignocellulosic Biomass
Round 1
Reviewer 1 Report
After reading the manuscript, I understood that the authors aimed to focused on the use of lignocellulosic biomass (LCB) as feedstock for thermophilic anaerobic digestion (TAD). However, in my humble opinion, the paper is too general, and so I would like to suggest the authors rewrite the paper to emphasize more on LCB. Advantages of TAD over mesophilic anaerobic digestion (MAD) should be clearly presented. I would also like to suggest the authors proofread the manuscript to improve readability and correct some grammatical errors. Please find the comments below that might be useful in rewriting the manuscript.
1. The title should be changed to reflect the focus of the paper. The authors might include “lignocellulosic biomass” in the title.
2. No information about LCB is given in the paper. A brief review on the compositions and potential of LCB as methane/biogas feedstock would enrich the manuscript.
3. Line 61-64: please cite reference(s) for the ranges of temperature used in TAD and MAD.
4. Line 65-66: the authors stated that higher degradation of the organic matter is due to improved solubility of the lipid at high temperature, what about other components in the feedstock, i.e., carbohydrate and protein? However, since the focus of this paper is placed on LCB, which contains negligible lipid content, the authors should revise the paper by focusing more on the major components in LCB.
5. Line 67-71: please cite reference(s) for “TAD is an advanced …. short time period.”.
6. Section 2 (Thermophilic anaerobic digestion: Key points): (1) I am not sure what key points are discussed in this section. (2) Please check the temperature ranges stated in Line 81-82 (hyperthermophilic AD is conducted at 80–120°C?). (3) the authors compared the pros and cons of MAD and TAD in Table 1, please give more information on each parameters compared. For example, please explain more on how TAD gives higher quality of digestate quality.
7. Line 100-102: the authors stated that Fig. 1 shows major AD steps occur during TAD. To my understanding, this is typical steps in AD. Is this different from what occurs during MAD? Also, what are (a) to (h) in the figure?
8. Section 3 (Impact of thermophilic inoculum AD) (Line 124-128): It is known that carrying out AD at high temperatures would facilitate hydrolysis of the feedstock, and thus improving the overall process. However, even if thermophiles could produce robust cellulolytic and hemicellulolytic enzymes, this does not mean pretreatment of LCB is not required. Although the authors have listed some reports investigating TAD of non-pretreated LCB, no experimental results are given, and so the efficiency of TAD of non-pretreated biomass is not clearly demonstrated.
9. Section 4 (Factors affecting the performance of thermophilic inoculum): the authors should give more extensive review on this aspect. Other factors, e.g., pH and temperature, should be reviewed.
10. Line 285-287: please explain how MEC and MFC could improve methane production, as well as overcome the organic and ammonia inhibitions.
11. Line 317-318: please explain how conductive materials (CMs) could accelerate and stabilize TAD process and methane production.
12. Section 3 (Techno-economic analysis): Although the authors present this section to help readers get an overall picture of TEA of TAD, in my humble opinion, this section is not relevant to the scope of this paper since there is no TEA reports on TAD of LCB. The authors might delete this section and rewrite the manuscript by focusing on other aspects, e.g., digestor design and engineering, process design for TAD, and scale-up approach.
13. Section 3 (Future perspective and conclusion): there is no future perspectives given in this section. Additionally, the efficiency of TAD over MAD was not clearly discussed in the paper (Line 404-406).
Author Response
Dear Reviewer
Thank you very much for your thorough review and sparing time out of your busy schedule. We have modified the changes as suggested by you. I believe, the manuscript has significantly revised after incorporating your suggestion and may be considered for publication. The changes have been made in track change and are visible.
Author Response File: 
Author Response.docx
Reviewer 2 Report
In this paper, the authors presented an interesting review on the relevant role of thermophilic microorganisms as inoculum in the anaerobic organic matter digestion and effective factors. Minor comments are listed as follows.
- Authors should provide a comparative table regarding the previous studied factors affecting the performance of thermophilic inoculum.
- At the beginning of each section, a brief explanation needs to be provided before the corresponding subsection begins.
- It is suggested to present a list of abbreviations in the appropriate part of the manuscript.
Author Response
Dear Reviewer
Thank you very much for your thorough review and sparing time out of your busy schedule. We have modified the changes as suggested by you. I believe, the manuscript has significantly revised after incorporating your suggestion and may be considered for publication. The changes have been made in track change and are visible.
Author Response File: Author Response.docx
Reviewer 3 Report
1. Please proofread the manuscript because there are some sentences that are confusing. For example the sentences on lines 47 until 49, and line 70.
2. Put a space before the citations.
3. Add a figure that summarizes the strategies to enhance the efficiency of TAD to make it easier for reader to graps the information.
Author Response
Dear Reviewer
Thank you very much for your thorough review and sparing time out of your busy schedule. We have modified the changes as suggested by you. I believe, the manuscript has significantly revised after incorporating your suggestion and may be considered for publication. The changes have been made in track change and are visible.
Author Response File: Author Response.docx
Reviewer 4 Report
For many organic waste streams thermophilic anaerobic digestion is a viable alternative to the more mesophilic anaerobic digestion. This review reiterates that point, which is essentially common knowledge in the field, but fails to go beyond that. It is essentially full of platitudes. The importance of a suitable inoculum is a case in point: this is mentioned, but nothing of value is added to that point. Similarly for the integration of bio-electrodes to enhance the process; mentioned but no real insights. In the introductory paragraphs the authors mention LCB but it is not clear how this sits in the present review.
The literature is poorly covered. Just one example: work from professors Schnuerer and Westerholm on TAD is not mentioned, but it should be.
This manuscript needs much more reflection, proper synthesis and a clear message / conclusions.
Selected minor comments (abstract only)
L16: LCB should be defined
L16: alleviation?
L16: advantages rather than advantageous features
L17: make rather than makes
L19: Sheds light? Hardly; discusses is more honest and even that is an over-statement.
L19: Pertinent? Not clear what this means in this context; an abstract should be clear! This phrasing is ambiguous.
L21: extended? Is also discusses (recent developments in) strategies to improve the efficiency…
L22-23: “Techno-economic feasibility of the process has also been discussed”. Really? Merely lip service; It is mentioned, but that is about it.
Author Response
Dear Reviewer
Thank you very much for your thorough review and sparing time out of your busy schedule. We have modified the changes as suggested by you. I believe, the manuscript has significantly revised after incorporating your suggestion and may be considered for publication. The changes have been made in track change and are visible.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Thank you for the responses to my comments. The manuscript has been revised well but there still are some points that need further clarification as follows.
1. Please check the temperature range for TAD. The temperature range for TAD is 50-60 °C (Line 110), but it is 45-80 °C in Line 170.
2. Section 3: according to the reference number [6], the temperature of 120 °C is not used in hyperthermophilic AD, but it was used for a thermal pretreatment of feedstock. Additionally, the reference number [6] does not report the temperature range for hyperthermophilic AD. Please check carefully.
3. Line 233-234: please add the unit of the bacterial count. Is it CFU/g?
Author Response
Dear Reviewer
Thank you very much for accepting our previous responses, and providing further comments and suggestions. We have incorporated the changes as suggested by you. I believe, the manuscript has significantly revised after incorporating your suggestion and may be considered for publication. The changes have been made in track change mode and are visible in the revised manuscript.
Reviewer-1
Thank you for the responses to my comments. The manuscript has been revised well but there still are some points that need further clarification as follows.
- Please check the temperature range for TAD. The temperature range for TAD is 50-60 °C (Line 110), but it is 45-80 °C in Line 170.
Reply: Thank you for the suggestion. The corrections have been made to remove the ambiguity.
- Section 3: according to the reference number [6], the temperature of 120 °C is not used in hyperthermophilic AD, but it was used for a thermal pretreatment of feedstock. Additionally, the reference number [6] does not report the temperature range for hyperthermophilic AD. Please check carefully.
Reply: Thank you for your suggestion. The correct reference has been cited and temperature range is modified.
- Line 233-234: please add the unit of the bacterial count. Is it CFU/g?
Reply: It’s correct, we have incorporated the unit.
Reviewer 4 Report
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Author Response
No comments received. However, the ms has been modified as per other reviewers comments.
Thank you
